Manage your subscription

Galileo teases out Jupiter's secrets

24 August 1996

By Justin Mullins

PICTURES from Galileo, the NASA spacecraft watching Jupiter, have
revealed huge thunderclouds on the giant planet. Their existence may help to
explain one of the strangest mysteries in the Solar System: why planetary winds
are more ferocious the further a planet is from the Sun.

Last week, NASA released photographs of Jupiter’s Great Red Spot, a gigantic
hurricane bigger than Earth that has raged for at least a century. The pictures
taken by Galileo show thunderclouds 100 kilometres wide rising 50 kilometres
above the other clouds, just like the anvil-head clouds associated with storms
on Earth.

“We always expected Jupiter to have thunderclouds but this is the first time
we’ve seen them,” says Andrew Ingersoll, a planetary scientist at the California
Institute of Technology who has analysed the images. “They’re huge by
terrestrial standards and create winds around 300 kilometres per hour.”

Ingersoll believes the storms are powered by a layer of steam that lies
unseen beneath the cloud tops. On Earth, thunderclouds form when evaporated
water condenses and releases heat. But a probe dropped into the Jovian
atmosphere last December failed to find water in the expected quantity.
Ingersoll says the images may help to solve this mystery. Jupiter’s storms seem
to be clustered to the north of the Great Red Spot. “Maybe that’s where the
water is,” he says.

Advertisement

Another possible explanation for the strong winds is that Jupiter may have
far fewer thunderstorms—which dissipate energy—than Earth, says
Ingersoll. “Without this dissipation the winds may just get stronger.” He says
that although it sounds paradoxical, the reason why there are fewer storms might
be because there is less energy from the Sun to trigger them.

Ingersoll’s ideas could explain why winds are even faster on the more distant
gas giants. For instance, Neptune receives only a twentieth the amount of
sunlight that falls on Jupiter, yet has winds nearly three times as fast.
“Perhaps Neptune has even fewer thunderstorms,” he says.

NASA also released new images of the Jovian moons Europa and Io. The images
of Europa are twice as clear as those taken by the Voyager spacecraft in the
1970s. They show that the moon’s icy surface may be older than scientists had
thought.

The surface of a moon can be dated by counting craters, which provide a
record of how many meteors have hit it. Voyager’s images show only a few craters
on the entire moon and this led scientists to believe that Europa’s surface may
be continually renewed by liquid water from beneath.

But the new pictures reveal many more craters along the moon’s
terminator—the dividing line between night and day where the craters cast
shadows that are easy to spot. The terminator is a few hundred kilometres wide
and about 2000 kilometres long, and represents only a small fraction of the
surface. “We’re seeing dozens of craters in this area,” says Ronald Greeley, a
planetary geologist at Arizona State University and a member of the imaging
team.

Greeley has not yet estimated the date of the surface. “We’re doing an
accurate crater count right now but the surface is certainly older than the
Voyager data suggest,” he says.

The question of whether liquid water lies beneath Europa’s icy
surface—and whether it might harbour some form of life—remains
unanswered. “There may be water but there may just be warmer ice,” says
Greeley.

Future images may help to resolve this question. Last week’s pictures have a
resolution of only 1.6 kilometres per pixel, but in December Galileo should send
pictures with a 22-metre resolution.

The new pictures of Io may help to explain why the moon has red polar caps.
Io is the most volcanically active body in the Solar System, heated by
gravitational squeezing as it orbits Jupiter. Both Voyager and Galileo pictures
show active volcanoes. But the recent images also show areas the size of Belgium
that have changed from deep red to a pale yellow since the Voyager flyby 17
years ago.

Scientists believe much of the debris in these areas is sulphur which comes
in many forms and colours. “If the red areas were short-chain sulphur molecules
of S3 and S4, the colour change may be due to phase transformations to more
stable, pale yellow forms of sulphur,” says Alfred McEwen of the Lunar and
Planetary Laboratory at the University of Arizona.

But Io’s poles are colder than the rest of the moon. “Any change would be
slower at colder temperatures,” he explains. This could be why the caps are
still red.